This invention relates to a catalyst and method for preparing the same, more particularly to a metal-supported catalyst for purifying the exhaust gas of an automobile and for combustion using a porous silicon carbide and/or a porous silicon nitride prepared by silicon-accumulated biomass such as rice hull and/or rice straw as a heat-resistant carrier and a method for preparing the same.
Since metals such as platinum, rhodium, palladium, etc. adsorb hydrogen and oxygen and the adsorbed hydrogen and oxygen have been activated, they have heretofore been utilized for the oxidation of ammonia, the preparation of hydrogen cyanide, the reforming of petroleum, etc. and as a reducing or oxidizing catalyst.
In recent years, accompanying the abrupt popularization of automobiles, air pollution due to their exhaust gas becomes remarkable so that purification of exhaust gas using a catalyst has been carried out by removing unburned hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NO.sub.x), etc. which are harmful substances in the exhaust gas of automobiles. As a catalyst for purifying the exhaust gas of automobiles, platinum group catalysts including platinum have mainly been used.
In respect to the reactions involved in purifying exhaust gas, the basic reactions are that HC and CO are converted to H.sub.2 O and CO.sub.2 by oxidation, respectively, and NO.sub.x is converted to N.sub.2 by reduction to make them harmless. Accordingly, it is required to use a oxidation-reduction binary catalyst. At present, an engine exhaust gas is balanced to the extent that either one of HC, CO and NO.sub.x can be purified in a theoretical air-fuel ratio, so that purification of the above three components has mainly been carried out by a ternary purification catalyst.
Rhodium (Rh) having high reductivity to NO.sub.x and platinum (Pt) or palladium (Pd) having high oxidation activity to HC and CO are used in combination as the ternary purification catalyst, and further oxides of nickel (Ni), iron (Fe), cobalt (Co), manganese (Mn), cerium (Ce) and zirconium (Zr) are used, but the main components thereof are platinum group catalysts such as Rh, Pt and the like.
These platinum group catalysts are practically used as pellets or in monolith form. The catalyst substances such as platinum are supported on the pellet compound of a piller or spherical shaped active alumina or on coating layer comprising an alumina stuck on the surface of the monolith compound of cordierite (2Al.sub.2 O.sub.3.5SiO.sub.2.2MgO) having a number of penetrated gas passages. However, since the pellet catalyst has a large heat capacity, it is inferior in heat properties. That is, when using a catalyst, it is required to heat a catalyst reactor to a predetermined temperature. But when the heat capacity of the catalyst is large, it takes a long time for heating the reactor so that it has a disadvantage that it does not work normally as a catalyst.
On the other hand, in the monolith catalyst, the melting point of the carrier is low so that it has problems in heat resistance, etc. Also, in the carrier comprising alumina which is used in both pellet or monolith catalysts, it has a problem with respect to the heat resistance under high temperature when used as a catalyst for a large scale engine of a large truck, etc. As described above, there are problems in that each of the conventional catalysts do not satisfy the requirements for functioning satisfactory as a catalyst.
Also, in combustion equipment such as boilers, in addition to CO, NO.sub.x, i.e., thermal NO.sub.x is generated by the reaction of nitrogen and oxygen in air at a temperature of 1500.degree. C. or higher whereby pollution is brought about. Thus, by using the above mentioned noble metal supported catalysts as combustion catalysts and utilizing their oxidizing properties and reductivities, research on catalytic combustion has been carried out to conduct perfect combustion at a low temperature which does not generate any thermal NO.sub.x or CO. This catalytic combustion is principally the same in the case of purification of exhaust gas from automobiles. In this catalytic combustion, it is desirable to conduct the combustion at such a high temperature as to not generate thermal NO.sub.x (1000.degree. to 1500.degree. C.) in order to increase the thermal recovery efficiency.
Accordingly, as a combustion catalyst, it has problems concerning the heat resistance thereof at high temperatures as in the temperatures used to purify exhaust gas from automobiles.